1. Field of the Invention
The present invention relates to an optical fiber preform, a method for producing the preform, and an optical fiber produced from the preform.
2. Description of the Background Art
In the case of a method known as the rod-in collapse method, a glass rod is inserted into a glass pipe to be heated together so that the two members can be unified. The method is suitable for producing a preform for an optical fiber having a complicated refractive-index profile, such as a dispersion-compensating fiber or a dispersion-shifted fiber.
With the rod-in collapse method, when a glass rod is inserted into a glass pipe, the center axis of the glass rod must be in precise alignment with that of the glass pipe. If the alignment is insufficient, the core noncircularity or eccentricity of the optical fiber preform increases. Even when the alignment is sufficient just after the glass rod insertion, the alignment may be disturbed during the heat-unifying process, increasing the core noncircularity or eccentricity as well. When the glass rod bends in the glass pipe, a similar drawback is incorporated.
When an optical fiber is produced by drawing an optical fiber preform having a high core noncircularity or eccentricity, the optical fiber inevitably has an increased core noncircularity or eccentricity. This drawback degrades the polarization mode dispersion of the optical fiber. The inferior polarization mode dispersion degrades the pulse shape of signals, preventing high-bit rate optical transmission.
An object of the present invention is to offer an optical fiber preform having a low core noncircularity and eccentricity for producing an optical fiber having an improved polarization mode dispersion, a method for producing the preform, and an optical fiber produced from the preform.
According to the present invention, the foregoing and other objects and advantages are attained by offering an improved rod-in collapse method as a method for producing an optical fiber preform. The method comprises the following steps:
(a) a preparing step in which a glass pipe, a glass rod, and a heat source are prepared;
(b) a diameter-reduced-portion-forming step in which an inner diameter of the glass pipe is reduced at two longitudinal locations for forming a first diameter-reduced portion and a second diameter-reduced portion;
(c) a glass-rod-inserting step in which the glass rod is inserted into the glass pipe;
(d) a glass-rod-fixing step in which the glass rod is fixed to the glass pipe at the first diameter-reduced portion; and
(e) a unifying step in which the glass pipe and the glass rod is heat-unified from the second diameter-reduced portion to the first diameter-reduced portion.
In this specification, the term xe2x80x9coptical fiber preformxe2x80x9d is used to mean a member to be drawn under the as-produced condition for producing an optical fiber or a member to be drawn for producing an optical fiber after being synthesized with a jacket over the circumference by an extra step such as the outside vapor deposition (OVD) method, the vapor phase axial deposition (VAD) method or the rod-in-tube method. Hereinafter, the latter member is referred to as an optical fiber preform intermediate.
The glass-rod-inserting step may comprise the following steps:
(a) inserting the glass rod into a protective pipe;
(b) inserting the protective pipe and the glass rod into the glass pipe; and
(c) withdrawing the protective pipe from the glass pipe leaving the glass rod in the glass pipe.
In the diameter-reduced-portion-forming step, a third diameter-reduced portion may be formed at a location between the first and second diameter-reduced portions and adjacent to the first diameter-reduced portion.
In the glass-rod-fixing step, the glass rod may be fusion-fixed to the glass pipe at the first diameter-reduced portion such that the first diameter-reduced portion is provided with a portion between the glass pipe and the glass rod through which a gas can pass at a flow rate of V satisfying the equation
Vxe2x89xa7{(dp/2)2xc3x97xcfx80xe2x88x92(dp/2)2xc3x97xcfx80}xc3x97v,
where dp is the inner diameter of the glass pipe at the portion other than the diameter-reduced portions,
dr is the diameter of the glass rod, and
v is the moving speed of the heat source relative to the glass pipe.
Each of the glass pipe and the glass rod facing each other at the first diameter-reduced portion may be made of highly pure silica glass or silica glass doped with fluorine, diphosphorus pentaoxide (P2O5), or chlorine (Cl). In the above description, the term xe2x80x9chighly pure silica glassxe2x80x9d is used to mean silica glass having a purity of at least 99.9 mol %.
The first and second diameter-reduced portions and the glass rod may have a dimensional relationship expressed as
dr less than Dxe2x89xa6dr+1(mm),
where D is the inner diameter of the first and second diameter-reduced portions.
The glass pipe and the glass rod may have a dimensional relationship expressed as
0.1(mm)xe2x89xa6(dpxe2x88x92dr)/2xe2x89xa63(mm).
The following two conditions may be added to the production method:
(a) The glass pipe prepared in the preparing step comprises an effective glass pipe to become a portion of the optical fiber preform, a first auxiliary glass pipe connected to one end of the effective glass pipe, and a second auxiliary glass pipe connected to the other end of the effective glass pipe.
(b) In the diameter-reduced-portion-forming step, the first diameter-reduced portion is formed at the first auxiliary glass pipe, and the second diameter-reduced portion is formed at the second auxiliary glass pipe.
The following two conditions may be added to the production method:
(a) The glass rod prepared in the preparing step comprises an effective glass rod at least one end of which is connected to an auxiliary glass member made of highly pure silica glass or silica glass doped with fluorine, P2O5, or Cl. In this case, the effective glass rod is to become a portion of the optical fiber preform.
(b) In the glass-rod-fixing step, the auxiliary glass member is fixed to the glass pipe at the first diameter-reduced portion.
In this case, the auxiliary glass member may be a third auxiliary glass pipe so that the effective glass rod can be connected to the third auxiliary glass pipe by inserting the effective glass rod into the third auxiliary glass pipe.
In accordance with an aspect of the present invention, a method for producing an optical fiber preform comprises the following steps:
(a) a preparing step in which:
(a1) an effective glass pipe to become a portion of the optical fiber preform, an auxiliary glass pipe, an effective glass rod to become a portion of the optical fiber preform, an auxiliary glass member made of highly pure silica glass or silica glass doped with fluorine, P2O5, or Cl. In this case, and a heat source are prepared;
(a2) a glass pipe is assembled by connecting the auxiliary glass pipe to one end of the effective glass pipe; and
(a3) a glass rod is assembled by connecting the auxiliary glass member to one end of the effective glass rod;
(b) a diameter-reduced-portion-forming step in which a first diameter-reduced portion is formed at the auxiliary glass pipe, and a second diameter-reduced portion is formed in the vicinity of the free end of the effective glass pipe;
(c) a glass-rod-inserting step in which the glass rod is inserted into the glass pipe;
(d) a glass-rod-fixing step in which the auxiliary glass member is fixed to the glass pipe at the first diameter-reduced portion; and
(e) a unifying step in which the glass pipe and the glass rod is heat-unified from the second diameter-reduced portion to the first diameter-reduced portion.
In accordance with another aspect of the present invention, an optical fiber preform having a core noncircularity of at most 1.5% is produced by a method comprising the following steps:
(a) a preparing step in which a glass pipe, a glass rod, and a heat source are prepared;
(b) a diameter-reduced-portion-forming step in which an inner diameter of the glass pipe is reduced at two longitudinal locations for forming a first diameter-reduced portion and a second diameter-reduced portion;
(c) a glass-rod-inserting step in which the glass rod is inserted into the glass pipe;
(d) a glass-rod-fixing step in which the glass rod is fixed to the glass pipe at the first diameter-reduced portion; and
(e) a unifying step in which the glass pipe and the glass rod is heat-unified from the second diameter-reduced portion to the first diameter-reduced portion.
In accordance with yet another aspect of the present invention, an optical fiber having a polarization mode dispersion of at most 0.15 ps/km1/2 in a wavelength band of 1,550 nm is produced from an optical fiber preform having a core noncircularity of at most 1.5% and is produced by a method comprising the following steps:
(a) a preparing step in which a glass pipe, a glass rod, and a heat source are prepared;
(b) a diameter-reduced-portion-forming step in which an inner diameter of the glass pipe is reduced at two longitudinal locations for forming a first diameter-reduced portion and a second diameter-reduced portion;
(c) a glass-rod inserting step in which the glass rod is inserted into the glass pipe;
(d) a glass-rod-fixing step in which the glass rod is fixed to the glass pipe at the first diameter-reduced portion; and
(e) a unifying step in which the glass pipe and the glass rod is heat-unified from the second diameter-reduced portion to the first diameter-reduced portion.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.